MTD/Montgomery wards slows and stop on incline

I have a Montgomery wards / MTD ( TMO -339398A) that slows to a stop when going up hills, or sometimes, its slows on when level.
Motor is running great.
I cant figure out is it belts ( which are new)/ pulleys. or the rear transmission.

As I go up hills and the mower starts to slow, I watch the belts, they stop turning and no movement ( blades still turning).
Happens on all speeds, worse on lower speeds.
Tensioner looks good, seems to be doing its job.
Where to start?
Thanks all.

Your mower uses 2 drive belts, and a double pulley betwwen them that has a floating center sheave, that moves to give you different speeds.
Most importantly is to have the correct belts, not just ones that appear to be the same lengths. The shape of the belt are unique, and the belts need to be right side up on some models.
But many times the problem is the moving parts in the system get rusty/sticky, and when the mower starts slowing, move the shift lever up or down without pressing the pedal down, and see if the mower speeds back up.
If so, try spraying some lube on the moving parts, including a light squirt in/on the center of the double pulley.

Do you use the clutch to shift gears?

Bring this one back, sorry for the delay.

The mower does not move when I try to push it in gear.
It seem to do fine on 6th and 7th gear, but when on a lower setting 4-5 it begins to move slowly and stops. I put new belts on last year, however, the were generic to match the size, not OEM belts, so maybe thats where the issue is.
I also plan to lubricate the pulleys, shafts, misc moving parts too.

I use the clutch to shift, but I likely could move up and down the gears without it currently. I'll need to try.